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Signals from seismic sensors left on the lunar surface by Apollo astronauts in the 1970s have revealed new insight into the moon's core, thanks to a fresh analysis using 21st century computing power.

The new study provides the first confirmation of layering of the moon's core and suggests that the moon, like Earth, has a solid inner core surrounded by a molten outer core, researchers said. But the moon's interior also has another layer of partially melted material – a ring of magma – around its outer core, the study found.

The findings come from data collected by four seismometers deployed on the moon by NASA astronauts between 1969 and 1972 during the space agency's six manned Apollo lunar landings. The seismometers kept working until 1977.

"The data itself has been in continual use since the Apollo era," said the study's lead author Renee Weber of NASA's Marshall Space Flight Center in Huntsville, Ala.

Most information about the composition of the center of the moon has been inferred from things such as its rotation, tidal distortion and magnetic field. However, there has been little hard data to draw on, researchers said.

"The moon's deepest interior, especially whether or not it has a core, has been a blind spot for seismologists," explained Ed Garnero, a professor at the School of Earth and Space Exploration at Arizona State University, who also participated in the study. "The seismic data from the old Apollo missions were too noisy to image the moon with any confidence."

Weber, Garnero and their colleagues took advantage of an innovative new technique — originally developed to analyze earthquake observations on Earth — to provide the first direct information about the moon's core.

In addition to the lunar interior's layering, the study also suggests that the moon's iron-rich inner core contains less than 6 percent of light elements such as sulfur, researchers said.

The new research is detailed in the Jan. 7 issue of the journal Science.

Blast from the lunar past
At the heart of the new moon core study is NASA's Apollo Passive Seismic Experiment[s1] , which Apollo astronauts set up on the lunar surface during all six moon landings, beginning with the historic Apollo 11 flight in July 1969.

Weber and her team did not use data from the Apollo 11 seismometer, but they did analyze the data from instruments deployed during the Apollo 12, 14, 15 and 16 flights. Another seismometer was also included in the final Apollo moon landing, Apollo 17, in 1972.

Information from the seismometers was sent back to Earth for almost eight years, until they were switched off in September 1977. Moonquakes and meteoroid impacts accounted for the majority of the data, though NASA did intentionally crashed rocket stages of several Saturn V moon rockets and the lunar landers used in the Apollo missions to help calibrate the seismic network.

Since then, scientists have worked to squeeze as much as possible about the moon's interior from the experiment, which is where Weber's team came in.

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At the moon's core
Weber and her colleagues started by examining the existing catalogue of lunar seismic signals. Of these, more than 6,000 were deep moonquakes, occurring about 435 miles (700 kilometers) below the surface, the study found.

These moonquakes originated from specific regions of the moon's interior, and each region produced repeatable seismic waveform signals, researchers said.

Because of this, scientists were able to employ a seismic waveform stacking technique, which piles data from the same source on top of each other. The result was more than 100 stacks of individual deep moonquake clusters.

On Earth, it is easier to see the different waveforms and seismic phases, but on the moon – which has a much more fractured surface from impacts – the seismic energy that reaches the surface is blurred and smeared.

Scientists had to use computing techniques to "unblur" the signals, then arrange the stacks of data to provide a clear measurement of the moon's interior, researchers said.

The density and size of the moon's core affects how long it would take for a moonquake to travel through it, Weber said. So the researchers could then predict when a hypothetical seismic wave would reach a certain point, which allowed them to compute the size and structure of the core with great precision.

Over three decades have passed since the last seismograph, but the data they provided is still vital to lunar studies. Still, Weber expressed a hope for more data to work with.

More seismic missions to the moon could provide scientists with the opportunity to further increase our knowledge of the lunar core, Weber said.

Southern stargazing

Stars, galaxies and nebulas dot the skies over the European Southern Observatory's La Silla Paranal Observatory in Chile, in a picture released on Jan. 7. This image also shows three of the four movable units that feed light into the Very Large Telescope Interferometer, the world's most advanced optical instrument. Combining to form one larger telescope, they are greater than the sum of their parts: They reveal details that would otherwise be visible only through a telescope as large as the distance between them.
(Y. Beletsky / ESO)
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A balloon's view

Cameras captured the Grandville High School RoboDawgs' balloon floating through Earth's upper atmosphere during its ascent on Dec. 28, 2013. The Grandville RoboDawgs’ first winter balloon launch reached an estimated altitude of 130,000 feet, or about 25 miles, according to coaches Mike Evele and Doug Hepfer. It skyrocketed past the team’s previous 100,000-feet record set in June. The RoboDawgs started with just one robotics team in 1998, but they've grown to support more than 30 teams at public schools in Grandville, Mich.
(Kyle Moroney / AP)
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Spacemen at work

Russian cosmonauts Oleg Kotov, right, and Sergey Ryazanskiy perform maintenance on the International Space Station on Jan. 27. During the six-hour, eight-minute spacewalk, Kotov and Ryazanskiy completed the installation of a pair of high-fidelity cameras that experienced connectivity issues during a Dec. 27 spacewalk. The cosmonauts also retrieved scientific gear outside the station's Russian segment.
(NASA)
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Special delivery

The International Space Station's Canadian-built robotic arm moves toward Orbital Sciences Corp.'s Cygnus autonomous cargo craft as it approaches the station for a Jan. 12 delivery. The mountains below are the southwestern Alps.
(NASA)
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Accidental art

A piece of art? A time-lapse photo? A flickering light show? At first glance, this image looks nothing like the images we're used to seeing from the Hubble Space Telescope. But it's a genuine Hubble frame that was released on Jan. 27. Hubble's team suspects that the telescope's Fine Guidance System locked onto a bad guide star, potentially a double star or binary. This caused an error in the tracking system, resulting in a remarkable picture of brightly colored stellar streaks. The prominent red streaks are from stars in the globular cluster NGC 288.
(NASA / ESA)
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Supersonic test flight

A camera looking back over Virgin Galactic's SpaceShipTwo's fuselage shows the rocket burn with a Mojave Desert vista in the background during a test flight of the rocket plane on Jan. 10. Cameras were mounted on the exterior of SpaceShipTwo as well as its carrier airplane, WhiteKnightTwo, to monitor the rocket engine's performance. The test was aimed at setting the stage for honest-to-goodness flights into outer space later this year, and eventual commercial space tours.

Red lagoon

The VLT Survey Telescope at the European Southern Observatory's Paranal Observatory in Chile captured this richly detailed new image of the Lagoon Nebula, released on Jan. 22. This giant cloud of gas and dust is creating intensely bright young stars, and is home to young stellar clusters. This image is a tiny part of just one of 11 public surveys of the sky now in progress using ESO telescopes.
(ESO/VPHAS team)
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Fire on the mountain

This image provided by NASA shows a satellite view of smoke from the Colby Fire, taken by the Multi-angle Imaging SpectroRadiometer aboard NASA's Terra spacecraft as it passed over Southern California on Jan. 16. The fire burned more than 1,863 acres and forced the evacuation of 3,700 people.
(NASA via AP)
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Where stars are born

An image captured by NASA's Spitzer Space Telescope shows the Orion Nebula, an immense stellar nursery some 1,500 light-years away. This false-color infrared view, released on Jan. 15, spans about 40 light-years across the region. The brightest portion of the nebula is centered on Orion's young, massive, hot stars, known as the Trapezium Cluster. But Spitzer also can detect stars still in the process of formation, seen here in red hues.
(NASA / JPL-Caltech)
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A long, long time ago...

This long-exposure picture from the Hubble Space Telescope, released Jan. 8, is the deepest image ever made of any cluster of galaxies. The cluster known as Abell 2744 appears in the foreground. It contains several hundred galaxies as they looked 3.5 billion years ago. Abell 2744 acts as a gravitational lens to warp space, brightening and magnifying images of nearly 3,000 distant background galaxies. The more distant galaxies appear as they did more than 12 billion years ago, not long after the Big Bang.
(NASA / NASA via AFP - Getty Images)
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Frosty halo

Sun dogs are bright spots that appear in the sky around the sun when light is refracted through ice crystals in the atmosphere. These sun dogs appeared on Jan. 5 amid brutally cold temperatures along Highway 83, north of Bismarck, N.D. The temperature was about 22 degrees below zero Fahrenheit, with a 50-below-zero wind chill.